The present invention relates, in general, to electrostatic spray guns, and more particularly, to an adapter for converting hand held airless, air assisted, or air-atomization spray guns to electrostatic or induction charging operations, or to a combination thereof, to provide improved spraying of, for example, electrically conductive or nonconductive coating materials such as high solid, water borne, metallic powder, or two-component paints, pyrolitic solutions, and the like.
Conventional airless, air assisted, or air atomization spray guns, such as those manufactured by Binks Manufacturing Company and others, incorporate a spray nozzle which includes liquid passageways and some mechanism for atomizing the liquid. The liquid, which may be paint, for example, flows under pressure through a central passage in the spray nozzle for discharge through a central orifice. This liquid flow is controlled, typically, by a fluid control needle valve located in the central passage, and the liquid is atomized as it is discharged. In an air assisted or air atomized spray gun, air passages are provided near the central fluid flow passage to assist in the atomization and to control the direction and flow pattern of the liquid particles. Thus, air under pressure coacts with the liquid ejected from the liquid outlet to further atomize the liquid and to impel the droplets outwardly away from the spray gun nozzle. Typically, the air flow is controlled by an air cap which surrounds the liquid outlet aperture. For example, the air cap may provide an annular air orifice surrounding the liquid outlet, may include additional air outlets around the air orifice, and may include a pair of forwardly projecting air horns which incorporate additional air nozzles directed generally inwardly toward the axis of the atomized spray to control its pattern. Typically, these air horns direct the atomized spray in a fan pattern to facilitate operation of the spray gun, with the air cap being rotatable on the spray gun to provide, for example, a vertical fan or a horizontal fan pattern.
When conventional spray guns of the foregoing types are used for spraying materials such as paint having a high solids content, metallic paints, and the like, problems are encountered, since such spray guns have low transfer efficiencies; for example, from 15 to 30 percent for an air-atomized paint spray, resulting in a great deal of wasted material. Improvements, including a greatly increased efficiency, have been obtained through electrostatic charging of the atomized coating material, such charging providing, for example, an efficiency in the range of 45 to 75 percent for electrostatic air atomized spray devices and from 90 to 99 percent for electrostatic rotary bell spray devices. However, even electrostatic devices present problems, particularly when spraying a conductive material such as water based paint, for it is necessary to electrically isolate such systems to prevent high voltages from endangering users or causing electrical discharges which could result in explosions. Various techniques have been provided for producing such isolation, such as isolating the paint supply from ground to prevent the high voltage that is being applied to the atomizer from leaking to ground through the paint supply line, utilizing a reverse charging process where the part being coated is placed at a high charge with the spray gun being at ground potential, or by utilizing an external charging system. Difficulties have been encountered in each of these systems, however, although external charging techniques utilizing, for example, a charging ring surrounding a rotary atomizer, have provided significant improvements in the application of water-borne coatings. The use of such a system has had limited use on high speed motion machines, and difficulties have been encountered in providing effective external charging of atomized coating particles with hand-held spray guns. Numerous attempts have been made to provide an external charging system for a hand-held spray gun that would effectively charge a wide variety of coating materials, including both electrically conductive and nonconductive materials, so as to produce a high transfer efficiency as well as to produce satisfactory spray coatings.
Most prior hand held electrostatic spray devices have in common a spray gun to which is mounted a high voltage electrode disposed adjacent the spray discharge point and carrying an electrical potential in the neighborhood of 50 to 85 kilovolts, and in some instances as high as 150 kilovolts. The voltage on this electrode creates a corona discharge condition, and the resulting electric field creates a region rich in ions through which the spray particles must pass. Some of these ions become attached to the spray droplets, producing electric charges on the particles which may then be directed toward a workpiece which is electrically grounded and which therefore attracts the charged particles. In addition, liquid contact with the metal spray nozzle or with a centrally located needle electrode also produces charges on the liquid and contributes to the overall charging of the particles.
Such corona discharge devices present numerous difficulties, principally as a result of the very high voltages required to produce effective operation. First of all, these high voltages usually are produced by separate electronic high voltage power supplies which are relatively large, heavy and expensive. Furthermore, because of the high voltages involved, the cable interconnecting the power supply and the spray gun charging electrode necessarily has to be heavily insulated and thus is bulky, relatively inflexible, and very expensive. The size and weight of the power supply and its cable substantially restricts the usefulness of the conventional corona effect spray gun both because of the difficulties encountered in handling and moving it, and the high cost.
Attempts have been made to overcome this problem, for example, through the use of turbine-driven voltage generators mounted in the spray gun and driven by the air flow to the nozzle. However, this requires extremely clean air, or the turbine becomes clogged, so large and expensive air filters are required. However, even these filters can become clogged and this reduces the air pressure to the gun. Other attempts have involved the use of high voltage ladder networks driven by conventional 110 voltage power connected through a relatively small cable. However, the very high voltages required in prior devices has caused problems due to dielectric breakdown caused, in part, by solvent erosion of the dielectric and potting compound materials. Such problems result in high costs, not only to meet quality control requirements to produce operable devices, but because of the resultant shortened lifetime of the equipment.
The use of high voltages in excess of 50 kV is hazardous not only because of the possibility of creating electrical arcs when the gun is moved near grounded objects, but because of the possible danger to the operator should he inadvertently touch the high voltage electrode. Finally, the high voltages used in such systems create a current flow of excess ions which travel to nearby objects, resulting in undesired charge build up on such objects that are not adequately grounded. The hazard of sparking and consequent fire exists when the operator or some other grounded object is brought close to such a charged object. Further, the migration of such charges causes an undesired build up of the charged spray particles on objects other than the workpiece. Attempts to control such hazards result in complex ground sensing circuits, which reduce current flow to prevent arcing, as described in U.S. Pat. No. 4,745,520.
It has been found that effective electrostatic spray coating can also be accomplished through the use of induction charging apparatus which eliminates the need for the very high voltages used in the corona discharge type of electrostatic charging. Induction charging of liquid particles in spray discharge devices has been accomplished by surrounding the discharged spray with a static electric field which has an average potential gradient in the range of about 5 to 30 kilovolts per inch, with the liquid being held at or near ground potential. In such devices, the spacing between the liquid and the source of potential is made sufficient to prevent an electrical discharge so that a capacitive effect produces a static field. This field induces on liquid particles produced within the field electrical charges having a polarity which is opposite to that of the applied voltage. The resulting charged particles can then be directed, for example, at an electrically grounded workpiece to provide a coating of the liquid on the workpiece. Such induction charging techniques have been found to be particularly useful in spray systems utilizing electrically conductive liquids such as water based paints, since the liquid supply can be electrically grounded. This is a considerable improvement over the above-described corona discharge and other high voltage spray devices which utilize a high voltage needle electrode in contact with the liquid. In such devices the liquid is at the same high voltage as the electrode, thereby requiring that the liquid supply be electrically isolated to prevent excessive current flow and to ensure the safety of the operator The lower voltages and the grounding of the liquid supply in an induction type of system eliminates the problems inherent in high-voltage isolated systems
An adapter to convert conventional nonelectrostatic spray guns as well as the high voltage corona discharge type of spray gun to induction charging is disclosed in U.S. Pat. No. 4,009,829. The described adapter is generally tubular and surrounds the spray nozzle of a conventional hand held or automatic spray gun of either the electrostatic or non electrostatic type. The forward end of the adapter extends beyond the end of the spray nozzle and is in the form of two diametrically opposed, forwardly extending lobes, each of which carries a charging electrode on its interior surface. A high DC voltage is applied between these electrodes and the liquid being sprayed to establish an electrostatic field within the charging zone defined by the device. The voltage applied is less than that required to cause corona discharge, but is sufficient to produce in the region near the liquid being sprayed a potential gradient of sufficient value to ensure that charges are induced on the particles sprayed from the nozzle.
The average potential gradient between the electrodes and the liquid supply in the device of the '829 patent is the average value of the voltage change per unit of radial distance between the axis of the liquid stream and the electrodes. The actual potential existing at any given point within the charging zone will depend upon the configuration of the electric field, and this will be influenced by factors such as the size and shape of the electrodes, the shape of the surface of the liquid stream, and the amount and location of the charge carried by spray particles within the zone. In the aforesaid U.S. Pat. No. 4,009,829, each charging electrode is in the form of a curved dielectric mounting plate carrying on its inner surface an electrically conductive metallic film, foil, or the like, and each mounting plate is secured to a corresponding lobe, but in spaced relationship to the lobe, to support the electrodes so as to define the charging zone. The curved electrodes are concentric to the axis of the spray nozzle to produce the desired electrostatic field configuration.
Similar adapters are illustrated and described in U.S. Pat. Nos. 4,073,002, 4,106,697, 4,186,886, 4,266,721, 4,313,968, 4,343,433, and 4,440,349, and in all of these patents the applicant herein is one of the named inventors. All of these patents disclose induction adapters either with or without corona assist. However, these devices generally require the use of a dielectric, such as plastic, air cap to prevent arcing or flashover between the electrodes and the spray gun. Such caps are more subject to abrasion and wear, and thus are less desirable than the conventional metal air cap. In addition, plastic air caps are more costly than metal caps, and are not available in the abundant variety of metal caps. Furthermore, prior spray gun devices required the use of high voltage cables or power supplies which are not only awkward to use, but present additional hazards to the user. Conventional high voltage electrostatic sprayers also present difficulties with certain coating materials. For example, conventional electrostatic sprayers produce lower concentrations and non-uniform distribution and/or orientation of metallic flakes in base coat applications, with the result that such coatings demonstrate poor color control and appearance when compared to conventional non-electrostatic air spray applied coatings. Furthermore, prior air electrostatic spray devices suffered from an excessive accumulation of droplets of the coating material on the spray gun. This is not only an inconvenience to the operator, but results in a loss of coating efficiency.